U.S. patent number 10,401,162 [Application Number 15/305,441] was granted by the patent office on 2019-09-03 for calibration of measurement probes.
This patent grant is currently assigned to RENISHAW PLC. The grantee listed for this patent is RENISHAW PLC. Invention is credited to Mark Cole, Stephen Edward Lummes.
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United States Patent |
10,401,162 |
Lummes , et al. |
September 3, 2019 |
Calibration of measurement probes
Abstract
A machine tool is provided with a toolsetting probe mounted on a
bed or table, and a workpiece-sensing probe which can be mounted in
a movable spindle. Both probes are calibrated by using them to make
measurements against each other. The arbitrary length of the
workpiece-sensing probe is used to calibrate the toolsetting probe,
rather than using a pre-calibrated artefact of known length mounted
in the spindle. A stylus disc of the toolsetting probe has a
pre-calibrated size or dimension, and the workpiece-sensing probe
is calibrated with respect to that. This obviates the need for
skilful manual calibration procedures using pre-calibrated
artefacts and manual measurement tools.
Inventors: |
Lummes; Stephen Edward (Stroud,
GB), Cole; Mark (Cam, GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
RENISHAW PLC |
Wotton-under-Edge, Gloucestershire |
N/A |
GB |
|
|
Assignee: |
RENISHAW PLC
(Wotton-under-Edge, GB)
|
Family
ID: |
53200225 |
Appl.
No.: |
15/305,441 |
Filed: |
April 23, 2015 |
PCT
Filed: |
April 23, 2015 |
PCT No.: |
PCT/GB2015/051204 |
371(c)(1),(2),(4) Date: |
October 20, 2016 |
PCT
Pub. No.: |
WO2015/162431 |
PCT
Pub. Date: |
October 29, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170045357 A1 |
Feb 16, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
Apr 23, 2014 [GB] |
|
|
1407178.1 |
Jan 22, 2015 [GB] |
|
|
1501100 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01B
5/012 (20130101); G05B 19/401 (20130101); G01B
21/042 (20130101); G05B 2219/37008 (20130101) |
Current International
Class: |
G01B
5/012 (20060101); G01B 21/04 (20060101); G05B
19/401 (20060101) |
Field of
Search: |
;33/502,503 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101427100 |
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May 2009 |
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CN |
|
101432592 |
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May 2009 |
|
CN |
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101622513 |
|
Jan 2010 |
|
CN |
|
102207731 |
|
Oct 2011 |
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CN |
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102483621 |
|
May 2012 |
|
CN |
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103328919 |
|
Sep 2013 |
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CN |
|
2243688 |
|
Nov 1991 |
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GB |
|
H04-063663 |
|
Feb 1992 |
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JP |
|
2528664 |
|
Aug 1996 |
|
JP |
|
H11-300580 |
|
Nov 1999 |
|
JP |
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2001-259966 |
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Sep 2001 |
|
JP |
|
2007/068912 |
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Jun 2007 |
|
WO |
|
2007/122362 |
|
Nov 2007 |
|
WO |
|
2013/083860 |
|
Jun 2013 |
|
WO |
|
Other References
Jul. 23, 2015 International Search Report issued in International
Patent Application No. PCT/GB2015/051204. cited by applicant .
Jul. 23, 2015 Written Opinion issued in International Patent
Application No. PCT/GB2015/051204. cited by applicant .
Jul. 5, 2018 Office Action issued in Chinese Patent Application No.
201580033849.1. cited by applicant .
Feb. 12, 2019 Office Action issued in Japanese Patent Application
No. 2016-564205. cited by applicant.
|
Primary Examiner: Guadalupe-McCall; Yaritza
Attorney, Agent or Firm: Oliff PLC
Claims
The invention claimed is:
1. A method of calibrating or datuming probes on a machine tool,
the machine tool comprising: a first part on which a workpiece is
mountable; a second part on which a tool is mountable, the first
and second parts being movable relative to each other; a control
which controls the relative movement of the first and second parts
of the machine tool; a toolsetting probe mountable on the first
part; and a workpiece-sensing probe mountable on the second part;
the method comprising: moving the first and second parts of the
machine tool relative to each other to bring the toolsetting and
workpiece-sensing probes into a sensing relationship with each
other in a probing direction, a dimension or position of a first
one of the probes being used to set one or more parameters of a
second one of the probes in the probing direction; and storing the
one or more parameters of the second probe in the control, wherein:
a dimension of the toolsetting probe is used to set one or more
parameters of the workpiece-sensing probe, the probing direction is
an X-axis and/or Y-axis direction of the workpiece-sensing probe,
and the one or more parameters of the workpiece-sensing probe
include an effective radius or diameter of a stylus tip of the
workpiece-sensing probe.
2. A non-transitory computer-readable medium storing a software
program for a control of a machine tool, the software program being
configured to cause the control of the machine tool to perform the
method according to claim 1.
3. A control for a machine tool, comprising a memory storing a
software program configured to cause the control to perform the
method according to claim 1.
4. A machine tool having the control according to claim 3.
5. The method according to claim 1, wherein a size of the dimension
of the toolsetting probe is pre-calibrated.
6. The method according to claim 1, wherein the dimension of the
toolsetting probe is on a deflectable stylus of the toolsetting
probe.
7. A method of calibrating or datuming probes on a machine tool,
the machine tool comprising: a first part on which a workpiece is
mountable; a second part on which a tool is mountable, the first
and second parts being movable relative to each other; a control
which controls the relative movement of the first and second parts
of the machine tool; a toolsetting probe mountable on the first
part; and a workpiece-sensing probe mountable on the second part;
the method comprising: moving the first and second parts of the
machine tool relative to each other to bring the toolsetting and
workpiece-sensing probes into a sensing relationship with each
other in a probing direction, a dimension or position of a first
one of the probes being used to set one or more parameters of a
second one of the probes in the probing direction; and storing the
one or more parameters of the second probe in the control, wherein:
a dimension of the toolsetting probe is used to set one or more
parameters of the workpiece-sensing probe, and a trigger signal
from the workpiece-sensing probe is used to set the one or more
parameters of the workpiece-sensing probe from the dimension of the
toolsetting probe.
8. The method according to claim 7, wherein a size of the dimension
of the toolsetting probe is pre-calibrated.
9. The method according to claim 7, wherein the dimension of the
toolsetting probe is on a deflectable stylus of the toolsetting
probe.
10. A method of calibrating or datuming probes on a machine tool,
the machine tool comprising: a first part on which a workpiece is
mountable; a second part on which a tool is mountable, the first
and second parts being movable relative to each other; a control
which controls the relative movement of the first and second parts
of the machine tool; a toolsetting probe mountable on the first
part; and a workpiece-sensing probe mountable on the second part;
the method comprising: moving the first and second parts of the
machine tool relative to each other to bring the toolsetting and
workpiece-sensing probes into a sensing relationship with each
other in a probing direction, a dimension or position of a first
one of the probes being used to set one or more parameters of a
second one of the probes in the probing direction; and storing the
one or more parameters of the second probe in the control, wherein:
a dimension of the toolsetting probe is used to set one or more
parameters of the workpiece-sensing probe, and a trigger signal
from the toolsetting probe is used to set the one or more
parameters of the workpiece-sensing probe from the dimension of the
toolsetting probe.
11. The method according to claim 10, wherein a size of the
dimension of the toolsetting probe is pre-calibrated.
12. The method according to claim 10, wherein the dimension of the
toolsetting probe is on a deflectable stylus of the toolsetting
probe.
13. A method of calibrating or datuming probes on a machine tool,
the machine tool comprising: a first part on which a workpiece is
mountable; a second part on which a tool is mountable, the first
and second parts being movable relative to each other; a control
which controls the relative movement of the first and second parts
of the machine tool; a toolsetting probe mountable on the first
part; and a workpiece-sensing probe mountable on the second part;
the method comprising: moving the first and second parts of the
machine tool relative to each other to bring the toolsetting and
workpiece-sensing probes into a sensing relationship with each
other in a probing direction, a dimension or position of a first
one of the probes being used to set one or more parameters of a
second one of the probes in the probing direction; and storing the
one or more parameters of the second probe in the control, wherein:
a dimension of the workpiece-sensing probe is used to set a datum
position or offset parameter of the toolsetting probe, and a
trigger signal from the toolsetting probe is used to set the datum
position or offset parameter of the toolsetting probe from the
dimension of the workpiece-sensing probe.
14. The method according to claim 13, wherein a length dimension of
the workpiece-sensing probe is measured and used to pre-position
the toolsetting and workpiece-sensing probes relative to each other
before they are moved into the sensing relationship.
15. The method according to claim 13, wherein the probing direction
is a Z-axis direction of the workpiece-sensing probe.
16. A method of calibrating or datuming probes on a machine tool,
the machine tool comprising: a first part on which a workpiece is
mountable; a second part on which a tool is mountable, the first
and second parts being movable relative to each other; a control
which controls the relative movement of the first and second parts
of the machine tool; a toolsetting probe mountable on the first
part; and a workpiece-sensing probe mountable on the second part;
the method comprising: moving the first and second parts of the
machine tool relative to each other to bring the toolsetting and
workpiece-sensing probes into a sensing relationship with each
other in a probing direction, a dimension or position of a first
one of the probes being used to set one or more parameters of a
second one of the probes in the probing direction; and storing the
one or more parameters of the second probe in the control, wherein:
a dimension of the workpiece-sensing probe is used to set a datum
position or offset parameter of the toolsetting probe, and a
trigger signal from the workpiece-sensing probe is used to set the
datum position or offset parameter of the toolsetting probe from
the dimension of the workpiece-sensing probe.
17. The method according to claim 16, wherein a length dimension of
the workpiece-sensing probe is measured and used to pre-position
the toolsetting and workpiece-sensing probes relative to each other
before they are moved into the sensing relationship.
18. The method according to claim 16, wherein the probing direction
is a Z-axis direction of the workpiece-sensing probe.
Description
FIELD OF THE INVENTION
This invention relates to the calibration of probes used on
position determination apparatus, such as machine tools.
DESCRIPTION OF PRIOR ART
It is known to use probes for measurement on machine tools. The
probes may be of the contact type, e.g. touch trigger probes which
issue a trigger signal upon contact with a workpiece which is to be
measured or with a tool which is to be set. A reading is taken from
the scales or other position transducers of the machine tool when
the trigger signal issues.
Two types of machine tool probe may be distinguished: One type
comprises probes which are used to measure workpieces, either for
workpiece setup prior to machining, or for inspection of workpieces
after or during machining. For example, the probe may be mounted in
a movable spindle of a machine tool such as a milling machine or
machining centre (a workpiece-sensing spindle probe). It is brought
into a sensing relationship with the workpiece by moving the
spindle. Another type of probe is used for setting cutting tools (a
toolsetting probe). For example, a toolsetting probe may be mounted
on a table of a milling machine or machining centre. A cutting tool
mounted in the movable spindle is set by bringing it into a sensing
relationship with the toolsetting probe.
A machine tool may be fitted with one of each type of probe.
Both types of probe require calibration before use, and periodic
re-calibration at intervals during use. This may involve
determining datum positions or offsets in the coordinate system of
the machine tool.
In the prior art, it is known to calibrate probes against artefacts
of known, pre-calibrated size, shape, form etc. Such artefacts
include things which have a known radius or diameter, such as
spheres, ring gauges or machined bores; and things which have a
known length, such as calibrated length bars, slip gauges, cutting
tools of known length, etc. Such artefacts are mounted in a
position on the machine tool which permits relative movement
between the artefact and the probe being calibrated.
For example, to calibrate a workpiece-sensing probe mounted in a
movable spindle of a machine tool, it is known in the prior art to
bring the probe into a sensing relationship with a pre-calibrated
artefact such as a sphere or ring gauge, which is fixed relative to
the machine table. This is because the probe will be moved relative
to the machine table, so it must be calibrated relative to the
machine table.
Conversely, to calibrate a toolsetting probe, it is known to mount
a pre-calibrated artefact such as a length bar or a cutting tool of
known length in the spindle, with a fixed relationship relative to
the spindle, because the toolsetting probe is being calibrated
relative to the position of the spindle. The length of the
pre-calibrated artefact is typically known relative to a gauge line
which may for example coincide with the nose of the spindle. The
spindle is moved in the Z-axis direction to bring the artefact into
a sensing relationship with the toolsetting probe.
Probes of the contact type may have a deflectable stylus with a
stylus tip that contacts a workpiece which is to be measured, or
which contacts a cutting tool which is to be set. A stylus tip may
be in the shape of a ball or (particularly for toolsetting) a
cylinder, disc, cube, etc. In addition to determining datum
positions using pre-calibrated artefacts as above, it is also known
to use such pre-calibrated artefacts to calibrate the size (e.g.
radius or diameter) of such stylus tips. The stylus tip component
may be supplied to a nominal size and the actual size is determined
through on-machine calibration. More commonly, even if the actual
stylus tip size is known accurately as supplied, the calibration
may determine an effective size dimension of the stylus tip (e.g.
radius or diameter) which differs from its actual size. In a touch
trigger probe, the effective size is typically smaller than the
actual size of the stylus tip, in order to calibrate out the
so-called "pre-travel" of the probe. Pre-travel is the constant
distance travelled during the fixed time delay between the instant
of actual contact with the workpiece or cutting tool, and the time
at which the probe issues a trigger signal.
It is believed that the machine tool builder Haas Automation Inc
has used a method of calibration which involves first calibrating a
toolsetting probe in a Z-axis direction, using a pre-calibrated
artefact such as a length bar or a cutting tool of known length
mounted in the spindle, as described above. This sets a Z-axis
datum position of the toolsetting probe. Then the effective Z-axis
length of a spindle-mounted workpiece-measuring probe is calibrated
by touching it against the top of the stylus tip of the calibrated
toolsetting probe. The length of the spindle-mounted probe is
determined from the trigger signal of the toolsetting probe.
Such known methods of calibration using pre-calibrated artefacts
are quite complicated, and it may be necessary to adopt careful
manual procedures e.g. using manual measurement tools such as dial
test indicators to set up the artefacts and to determine the
relationship to the axis of rotation of the spindle, etc. The known
methods therefore require skilled personnel and take a considerable
time (e.g. 30 minutes) during which time the machine tool is
unproductive. For these reasons, it is common in practice that the
calibration is not performed well, and/or that re-calibration at
periodic intervals is neglected. This results in poor measurement
accuracies and poor quality machined workpieces.
SUMMARY OF THE INVENTION
The present invention provides various methods of calibrating or
datuming probes on a position determining apparatus such as a
machine tool, the apparatus comprising a first part and a second
part which are movable with respect to each other; a toolsetting
probe mountable on the first part; and a workpiece-sensing probe
mountable on the second part.
In a first such method according to the invention, the toolsetting
probe is calibrated or datumed from relative movement between it
and the workpiece-sensing probe. The relative movement brings the
workpiece-sensing probe into a sensing relationship with the
toolsetting probe, and a calibration or datum value for the
toolsetting probe is taken when it reaches that sensing
relationship. A dimension or size of the workpiece-sensing probe
may thus be used to calibrate or datum the toolsetting probe. That
may be distinguished from the prior art use of a pre-calibrated
artefact such as a length bar or cutting tool having a known length
relative to a gauge line on the second part (e.g. on a spindle).
The dimension or size of the workpiece-sensing probe may be
arbitrary.
In this first method, the probes may be contact probes, for example
with deflectable workpiece-contacting or tool-contacting styli. The
sensing relationship may be a contact between them. The relative
movement may be in an axial direction (or a Z-axis direction) of a
tool-holder or spindle of the apparatus, to which the
workpiece-sensing probe is mounted.
In a second such method according to the invention, a portion or
feature of the toolsetting probe has a pre-calibrated size or
dimension, and the workpiece-sensing probe is calibrated or datumed
with respect to that pre-calibrated size or dimension, e.g. by
relative movement between the probes which brings them into a
sensing relationship. The portion or feature with the
pre-calibrated size or dimension may be on a movable part of the
toolsetting probe. That may be distinguished from the prior art use
of a pre-calibrated artefact such as a sphere or ring gauge of
known size, mounted on the bed or table of a machine tool.
In this second method, the toolsetting probe may be a contact
probe, and the portion or feature with the pre-calibrated size or
dimension may be on a deflectable stylus of the probe. The
workpiece-sensing probe may also be a contact probe, e.g. with a
deflectable workpiece-contacting stylus. The sensing relationship
may be a contact between them. The relative movement may be one or
both directions transverse to a tool-holder or spindle of the
apparatus, to which the workpiece-sensing probe is mounted.
In a third such method according to the invention, the
workpiece-sensing probe and the toolsetting probe are each
calibrated or datumed relative to the other. A portion or feature
of one of the probes may have a pre-calibrated size or dimension,
and the other probe is calibrated or datumed with respect to that
pre-calibrated size or dimension, e.g. by relative movement between
the probes which brings them into a sensing relationship. The
portion or feature with the pre-calibrated size or dimension may be
on a movable part of the respective probe. This third method may
incorporate further preferred features from either or both of the
first and second methods
The invention also provides software programs configured to perform
any of the above methods when run in a machine tool control. It
also provides machine tools and machine tool controls configured to
perform such methods.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, with
reference to the accompanying drawings, wherein:
FIG. 1 shows operative parts of a machine tool
diagrammatically;
FIGS. 2-6 illustrate probes in the machine tool of FIG. 1,
performing steps of a preferred method according to the invention;
and
FIG. 7 is a flowchart of that preferred method.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows operative parts of a machine tool, comprising a bed 10
to which a workpiece may be clamped, and a rotatable spindle 12 in
which a cutting tool may be mounted. The spindle 12 is movable in
three axis dimensions X, Y, Z relative to the bed 10. This movement
is driven by X, Y, Z motors 30, and is measured by X, Y, Z encoders
or scales 32 (or other position transducers), which provide
position feedback in a servo loop. The X, Y, Z movement is
controlled by a program running in a control 18 of the machine
tool, which typically also controls the rotary movement of the
spindle 12. The control 18 may be a conventional CNC controller or
a separate computer. Although a movable spindle and a fixed bed are
shown, as an alternative the fixed bed may be substituted by a
movable table, providing the movement in one or more of the
dimensions relative to the spindle.
A toolsetting probe 14 is clamped to the bed 10. It may be used for
setting cutting tools. A spindle probe 16 is mounted in the spindle
12, and is exchangeable with cutting tools by a conventional tool
changer (not shown). The spindle probe 16 may be used for workpiece
setup and/or for measuring workpieces. The probes 14, 16 provide
signals to the control 18 of the machine tool via an interface 19.
The communication between the spindle probe 16 and the interface 19
and control 18 may be provided via a wireless transceiver module
20, such as an optical or infrared or radio module, or via an
inductive coupling. The toolsetting probe 14 may also communicate
with the control via a wireless transceiver module if desired.
In the present example, the probes are both of the contact type.
The spindle probe 16 has a stylus 24 which is biased by a spring
into a rest position in the probe body and which deflects against
the force of the spring when a workpiece-contacting tip 28 of the
stylus contacts a workpiece. Likewise, the toolsetting probe 14 has
a stylus 22 (having a cranked configuration in this example) which
is biased into a rest position in the body of the probe 14 and
which deflects when contacted by a cutting tool. The
tool-contacting stylus tip 26 of the stylus 22 may be in any
conventional form such as a cube, disc, cylinder or sphere. It has
a known, pre-calibrated dimension which is preferably certified
when supplied. It may have been measured on a separate measuring
apparatus such as a coordinate measuring machine. This is in
contrast to known stylus tips for such probes, which may have a
stated nominal diameter or other dimension when supplied, but the
actual or effective diameter (or other dimension) is determined as
part of the calibration process on the machine tool. In the present
example the stylus tip 26 of the toolsetting probe is in the form
of an accurately ground disc with a known, certified pre-calibrated
diameter.
The probes 14, 16 are suitably touch trigger probes which issue a
trigger signal when their deflectable styli contact or are
contacted by a workpiece or cutting tool. This causes the control
18 to latch the outputs of the X, Y and/or Z scales 32, as
appropriate, giving readings which indicate the instantaneous
relative X, Y and/or Z position of the spindle 12 and bed 10. If
desired, instead of a touch trigger probe, the probe 16 may be an
analogue or scanning probe providing X, Y, Z outputs measuring the
amount of deflection of the stylus 24 as a result of contact with a
workpiece surface.
Before use, it is required to calibrate or "datum" the probes 14,
16. This is performed by calibrating them against each other, as
shown in FIGS. 2-6 and in the flow chart FIG. 7.
In an optional preliminary step 40, the user estimates or measures
the approximate distance from the nose of the spindle 12 to the
stylus tip 28 of the spindle probe 16, e.g. with a ruler. The
result is manually entered into the control 18, and is simply a
positive offset which is used to enable the control to pre-position
the probe 16 in the Z direction, with the stylus tip 28 a short but
arbitrary distance above the disc 26 of the toolsetting probe 14.
The exact height of the spindle probe stylus tip 28 above the
toolsetting probe disc 26 is not important, so the measurement in
this step 40 need not be performed accurately.
Alternatively, in the optional preliminary step 40 the user may
estimate or measure the distance from the bed 10 of the machine
tool to the stylus tip 28 of the spindle probe 16. This is manually
entered into the control 18 as a negative offset, which is used for
the same purpose.
If the above pre-positioning step 40 has been omitted, the spindle
12 can be pre-positioned manually, with the spindle probe at an
arbitrary height above the disc 26. However, this may take longer
because of the manual positioning, and because in the next step 42
(discussed below) the spindle may need to move a greater distance
at one or more relatively slow feed rates.
Once pre-positioned above the disc 26, as shown in FIG. 2, the
control 18 runs a software macro program with the following steps.
The program may be loaded and stored in the control 18 from any
suitable machine-readable storage medium, e.g. a tape, CD-ROM disc
or USB memory stick.
In step 42, the toolsetting probe 14 is switched on (by enabling it
in the interface 19) and the spindle 12 is fed downwards in the Z
direction from the position shown in FIG. 2, so that the spindle
probe 16 is brought into contact with the disc 26 of the
toolsetting probe 14. When the stylus tip 28 of the spindle probe
16 contacts the disc 26 of the toolsetting probe 14, the stylus 22
deflects and the toolsetting probe 14 issues a trigger signal to
the control 18. This latches the Z output of the Z scale 32, and
stops the movement in the Z direction. The latched Z reading from
the scale is stored in the control 18 as a Z axis trigger position
(in the coordinate system of the machine tool) for subsequent use
as a datum position or offset during measurements.
This Z movement in step 42 should preferably take place at a feed
rate corresponding to that which will subsequently be used during
normal measurements. This makes it possible to calibrate out the
so-called "pre-travel" (the constant distance travelled during the
fixed time delay between the instant of actual contact with the
disc 26 and the time at which the Z scale 32 is latched). If in
practice measurements are to be made at more than one feed rate,
then each feed rate will involve a different amount of pre-travel.
So the step 42 should be repeated at each feed rate, and
corresponding Z axis trigger positions recorded in the control 18.
For example, it may be desired to set the Z length of a cutting
tool both when it is rotating and when it is static (not rotating).
Typically a lower feed rate would be used when rotating and a
higher feed rate when not rotating. In this case, step 42 is
repeated for both feed rates.
The above step 42 sets a Z-axis datum position or offset for the
toolsetting probe 14, against which future measurements of the
Z-axis length of cutting tools will be referenced. This datum
position or offset has been set using the Z-axis length of the
spindle probe 16 with its stylus 24. As already mentioned, this
Z-axis length of the spindle probe is arbitrary--it need not be
known. Even if it is measured in the optional step 40, this
measurement can be very approximate and inaccurate, since the
purpose is simply to pre-position the probe. This is in contrast to
prior art methods, where the Z-axis height of the toolsetting probe
is first calibrated or datumed against a length bar or cutting tool
which is held in the spindle and has an accurately known length
relative to a gauge line on the spindle; and where the Z-axis
length of the spindle probe is then calibrated by touching it
against this pre-calibrated toolsetting probe. In such prior art
methods, the length bar or cutting tool of known length must be
carefully set up, with accuracy commensurate with the required
accuracy of the subsequent toolsetting and workpiece
measurements.
Next, the toolsetting probe 14 is switched off and the spindle
probe 16 is switched on (by disabling one and enabling the other in
the interface 19). In step 44, the control 18 is programmed to move
the spindle 12 so that the stylus tip 28 of the probe 16 is brought
into contact with the sides of the disc 26 from both +X and -X
directions (FIGS. 3 and 4), and also from both +Y and -Y directions
(not shown). At this stage, the precise X-Y position of the disc 26
is not known, and so these movements do not take place exactly on
the X and Y diameters of the disc 26. The movements take place
while the probe 16 is static (not rotating) in the spindle 12.
Thus, when the stylus tip 28 of the spindle probe 16 contacts the
disc 26 of the toolsetting probe 14, the stylus 24 deflects and the
spindle probe 16 issues a trigger signal to the control 18. When
the trigger signal is received, readings are taken from both the X
and Y scales 32, at each of the four touch points in the +X, -X, +Y
and -Y directions. This step 44 enables the control 18 to calculate
more accurately the X and Y positions of the X, Y diameters of the
disc 26. It can then pre-position the probe 16 on the X and Y
diameters of the disc 26 for the next step 46. However, this step
44 could be omitted if the probe 16 can be positioned with
sufficient accuracy without it.
Step 44 determines the X, Y position of the toolsetting stylus disc
26 relative to the stylus ball 28 of the spindle probe 16. However,
this will not necessarily give the position of the disc relative to
the axis of rotation of the spindle 12, since the probe 16 may
mounted in the spindle with its stylus tip 28 off the axis of
rotation.
In step 46, therefore, the probe 16 is rotated in the spindle 12,
as indicated by arrows R in FIGS. 5 and 6. While continuously
rotating, it is again brought into contact with the disc 26 along
the diameters determined in step 44, from both +X and -X directions
(as shown), and also from both +Y and -Y directions (not shown).
The control 18 latches the X-Y scale readings for each contact, on
receipt of the trigger signal from the probe 16. Even if the probe
16 is mounted in the spindle 12 with its stylus tip 28 off the axis
of rotation, the feed rate is set sufficiently low, relative to the
speed of rotation, that these X-Y scale readings are not affected
by any such off-axis mounting. This step therefore enables the
control 18 to determine the position of the disc 26 in the X-Y
plane, relative to the true axis of rotation of the spindle 12. The
centre of the disc in the X direction is determined as the midpoint
between the readings taken in the +X and -X directions. The centre
of the disc in the Y direction is determined as the midpoint
between readings taken in the +Y and -Y directions.
Alternatively, step 46 may determine the position of the disc 26
relative to the true axis of rotation of the spindle 12 as follows.
Rather than continuously rotating the spindle probe 16 while
bringing it into contact with the disc, each of the readings in the
+X, -X, +Y and -Y directions is taken with the probe 12 static (not
rotating). However, between the +X and -X readings, the probe 12 is
rotated through 180.degree.. Likewise, it is also rotated through
180.degree. between the +Y and -Y readings. As above, the control
18 then uses the midpoints between the readings to determine the
true position of the disc 26 in the X-Y plane, relative to the true
axis of rotation of the spindle 12.
This step 46 could be performed without spindle rotation if there
is confidence that the stylus tip 28 is centred on the axis of
rotation. However, to have such confidence would probably require
manual checking and adjustment, and it is advantageous to avoid
that.
The above steps calibrate the toolsetting probe 14. We now have an
artefact (the pre-calibrated disc 26) which has a known size and
known position in space. That is, the diameter of the disc 26 is
known from its calibration value, and its X-Y position in the
coordinate system of the machine tool has been determined as above,
relative to the axis of rotation of the spindle 12. The spindle 12
can be aligned with the centre of the toolsetter disc 26. The
toolsetting probe 14 also has a Z-axis datum position or offset as
a reference when setting the Z-axis length of cutting tools against
the toolsetting probe 14. These values, or offsets or datum values
derived from them (e.g. relating to the +X, -X, +Y, -Y contact
positions on the perimeter of the stylus disc 26) are stored in the
control 18, for use when making subsequent measurements in order to
set cutting tools. However, it should be noted that when the
toolsetting probe 14 is triggered by a cutting tool, it will be
subject to pre-travel. This can be calibrated out by adding or
subtracting a constant distance determined from experience, e.g. 40
.mu.m, to the +X, -X, +Y, -Y contact positions. Suitably the stored
calibration values or offsets are adjusted by this experience
value.
Next, the spindle probe 16 is calibrated. In step 48, the stylus
tip 28 is again touched against the toolsetter disc 26 from both
the +X and -X directions, and also from both +Y and -Y directions,
with the probe 16 static (not rotating). This time, unlike FIGS. 3
and 4, the spindle 12 is aligned with the diameters of the disc 26
in the X and Y axes, as found in step 46. The X and Y scale
readings are respectively latched when the probe 16 issues a
trigger signal as a result of these touches. The purpose of this
step is to obtain stylus offsets for use when measuring workpieces
by probing them with the spindle probe 16, which are stored in the
control 18. This calibrates the spindle probe 16 for X-Y
measurements, at least in part.
Finally, the calibration of the spindle probe 16 is completed in
step 50. This determines the effective radius or diameter of the
stylus ball 28 of the spindle probe 16, and its position relative
to the disc 26 of the toolsetting probe 14. It will be recalled
that, in general, the mounting of the probe 16 in the spindle 12
may be such that the position of the stylus ball 28 does not
coincide with the axis of rotation of the spindle 12. The
measurements in step 48 take place with the axis of rotation
(rather than the stylus ball 28) aligned with the disc 26, which
may not enable the radius and position of the stylus ball 28 to be
determined sufficiently accurately.
In step 50, therefore, once again the stylus tip 28 is touched
against the toolsetter disc 26 from both the +X and -X directions,
and also from both +Y and -Y directions, with the probe 16 static
(not rotating). The resulting trigger signals from the spindle
probe 16 are used to latch the X, Y scales 32 as previously.
Assuming the control 18 takes the +X and -X readings first, it then
determines the midpoint between these two readings. This gives the
centre of the stylus ball 28 relative to the toolsetting disc 26,
in the X direction. Next, using this information, it moves the
spindle 12 to align the centre of the stylus ball 28 with the Y
diameter of the disc 26, and takes the +Y and -Y readings. The
midpoint between the +Y and -Y readings likewise gives the centre
of the stylus ball 28 relative to the toolsetting disc 26, in the Y
direction. Furthermore, the diameter of the stylus ball 28 is
calculated by subtracting the known, pre-calibrated diameter of the
disc 26 from the difference between the +Y and -Y readings. The
effective radius of the stylus ball 28 is calculated by dividing
this diameter by two.
The feed rate used in steps 48 and 50 should preferably correspond
to that which will be used during probing measurements on
workpieces. As described for step 42, this calibrates out the
pre-travel (the constant distance travelled during the fixed time
delay between the instant of actual contact with the disc 26 and
the time at which the X and/or Y scale 32 is latched). The control
18 thus takes account of the pre-travel, evaluating the so-called
"electronic" radius or effective radius of the stylus ball 28,
rather than its physical radius. If more than one feed rate will be
used during probing measurements, steps 48 and 50 should preferably
be repeated at each feed rate.
Touch trigger probes commonly have different values of pre-travel
when probing in different directions (vector angles) in the X-Y
plane. Consequently, if different X-Y probing directions are to be
used, the step 50 may be repeated for each of these directions, not
only for the +X, -X, +Y and -Y directions. This gives an
"electronic" or effective radius of the stylus ball corresponding
to each direction (each vector angle). For example, the control may
calculate and store up to 12 effective stylus radii, corresponding
to 12 vector angles.
The resulting values for the radius and position of the stylus ball
are used by programs in the control 18 to correct subsequent
measurements of workpieces.
The method as described may be used with probes 14, 16 whose styli
are biased into the rest position with fixed spring rates and
spring pre-loads, set by their design or during their manufacture.
It may also be used with probes whose spring rates or pre-loads are
user-adjustable. The effective X-Y axis spring rates and/or spring
pre-loads of the probes 14, 16 are preferably set relative to each
other, such that in steps 44, 46, 48 and 50 the stylus 24 of the
spindle probe 16 deflects in the X-Y directions before the stylus
22 of the toolsetting probe 14. This may be achieved if the stylus
22 of the toolsetting probe 14 is biased into its rest position
with a significantly larger effective X-Y pre-load than the stylus
24 of the spindle probe, so that the toolsetter stylus 22 remains
in its rest position during the initial X-Y deflection of the
stylus 24 and triggering of the spindle probe 16. Alternatively,
the respective effective X-Y spring rates of the probes may be such
that the toolsetter stylus 22 deflects by only a relatively small
amount as the spindle probe 12 triggers. Such a small amount of
movement of the toolsetter stylus could affect the determination of
the effective radius of the spindle probe stylus tip 28 in step 50.
However, this can be compensated by subtracting a small constant
value determined by experience (e.g. 2 .mu.m) from the effective
radius of the stylus tip 28.
On the other hand, the effective Z axis spring rates and/or
pre-loads of the probes 14, 16 are preferably set relative to each
other such that in step 42, the stylus 22 of the toolsetting probe
14 deflects in the Z direction before the stylus 24 of the spindle
probe 16.
To determine the effective spring rates or pre-loads in a given
direction, the actual spring rates and pre-loads should be
considered in the light of the leverage resulting from the lengths
and configurations of the styli 22, 24.
As described above, trigger signals from the toolsetting probe 14
are used for the Z-axis calibration. However, trigger signals from
the spindle probe 16 may be used instead, e.g. re-arranging the
relative spring rates and pre-loads of the two probes. Likewise,
trigger signals from the toolsetting probe 14 may be used for the
X-Y calibration, rather than trigger signals from the spindle probe
16. Again, it would be desirable to re-arrange the relative spring
rates and pre-loads of the probes.
Rather than switching the probes 14, 16 on and off as described
above, both could be enabled together, and the trigger signal taken
from whichever probe triggers first. To achieve this, the interface
19 may comprise hardware or software which performs a logical OR
function on the inputs from the two probes.
Unlike pre-calibrated artefacts in the prior art, it will be
appreciated that the disc 26 is not in a fixed position relative to
the machine tool bed or table 10. In fact, it is designed to move
in any of the directions X, Y, Z when contacted by a cutting tool
during normal toolsetting operations. However, the spring rates or
pre-loads of the probes 14, 16 are set as above, such that any
movement of the disc 26 during steps 46, 48 or 50 is small relative
to the desired accuracy of the calibration. Alternatively, if such
movement is significant, then the measured position of the disc can
be compensated by subtracting or adding constant compensation
values to the readings, determined in the light of experience.
Rather than using a pre-calibrated dimension on the stylus 26 of
the toolsetting probe 14, it is possible to use a pre-calibrated
dimension provided elsewhere on the toolsetting probe, for example
the diameter of a portion of its housing.
With the exception of the optional first step 40, the above method
can proceed automatically under program control, without
intervention from a skilled technician. It can be completed in less
than two minutes, compared to prior art methods which take 30
minutes. Even the first step 40 is easy and requires little skill,
since it is just an approximate measurement with a ruler. The
probes are calibrated simply by probing each of them with the
other. There is no need for ancillary calibration artefacts such as
pre-calibrated spheres, ring gauges, length bars or tools of known
dimensions. Nor is there a need to carefully ensure that such
artefacts are accurately set up or accurately centred on the
rotational axis of the spindle, using manual measuring equipment
such as dial test indicators.
As described above, the steps 40, 42 for calibration in the Z-axis
are performed before the steps 44-50 for calibration in the X-Y
axes. However, the Z-axis calibration could equally be performed
after the X-Y calibration. It is also possible to perform the
Z-axis calibration without the X-Y calibration, or vice versa.
The above example may be modified for application to other machine
tool configurations, such as a lathe. In a lathe, the probe 16 may
be mounted in a movable turret which also holds cutting tools. If
the probe 16 is not mounted in a rotatable spindle, then step 46
must be performed without rotation. The toolsetting probe 14 may be
mounted on a fixed bed. Alternatively, it may be mounted on a
movable arm such as one of the HP series of toolsetting arms
available from the present applicants Renishaw plc, which enables
it to be moved into an operative position when calibration and
toolsetting are required, and moved out of the way during normal
machining.
* * * * *